The present invention refers to a hydraulic cylinder for telescopic arms, especially arms consisting of several telescopic sections which expand and retract sequentially, with each section housing a cylinder which begins to operate at the end of the run of the cylinder of the immediately previous or following section, in the stages of extension and retraction, respectively.
Telescopic arms with the above-described composition form part, for example, of hydraulic cranes with a telescopic arm in which each section does not extend or retract until the preceding arm cylinder has arrived at the end of its hydraulic run. Likewise, the retraction of any cylinder cannot occur until the cylinder which is being retracted reaches its position of complete retraction.
Cylinders are known which satisfy this principle for the expressed purpose. In this sense, the European patent EP0562096 may be mentioned, related to a loading crane with a jib or arm comprising several sectors which may be telescopically coupled, inserting in each other, each sector being fitted with a hydraulic cylinder by means of which the sequential extension or retraction of the different sections is achieved, such that each section does not start its extension until the cylinder housed in the preceding section has arrived at the end of its hydraulic run, with retraction occurring in a similar manner. For this reason each section is provided with a stop which activates a valve when the arm reaches its retracted position, allowing passage of the hydraulic liquid towards the cylinder of the following arm. This valve is located in the closed end of the hydraulic cylinder of each arm.
Said composition may present operation problems and basically demand a complicated composition due to the fact that the valve which controls the extension or retraction in the different arms consists of a flow valve for hydraulic fluid.
The object of the present invention is to eliminate the aforementioned problems by means of a cylinder of simple composition and operation, and whose retraction or extension operations are controlled by means of a device which does not control nor is in contact with the hydraulic fluid for cylinder activation.
The cylinder of the invention has a known general composition, consisting of a cylindrical sleeve closed by end covers, an internal piston delimiting expansion and retraction chambers, a tubular shank associated to the piston which protrudes through the rear cover and is closed by a head integrally joined to said shank and passage orifices for the expansion and retraction chambers.
In the cylinder of the invention, the cover closing the expansion chamber includes the passage hole to this chamber, which enters said chamber through two mouths provided with as many anti-return valves which open in opposite directions. One of these mouths is located in a centred position and inside it a conduit is fixed in the cover, which co-axially runs through the chamber and reaches the piston.
A tube runs co-axially through the tubular shank of the piston, reaching a front housing located in the anterior base of said piston. The wall of this tube has orifices in its wall near the aforementioned mouth, and in addition the tube may be partially displaced with respect to the piston by the action of the following section of the telescopic jib in its retraction position. This tube has a greater diameter than the conduit which is integrally joined to the central mouth of the expansion chamber cover, such that when the piston is displaced in the retraction direction, said tube co-axially receives the conduit joined to the cover. The tubular shank and the tube running inside the latter define, in the external head of the shank, two coaxial chambers provided with a mouth for intercommunication to the expansion and retraction chambers of the cylinder housed in the following section of the telescopic arm.
With the above described composition, when the piston reaches its maximum expansion run the aforementioned tube and conduit remain extended, while during the retraction stage, the conduit integrally joined to the cover is gradually introduced in the tube which runs through the tubular shank. The conduit integrally joined to the cover has passage orifices in its wall, near the end which reaches the housing of the anterior base of the piston. Between these orifices and the free end, this conduit is provided with a bushing which may move between an anterior position in which it closes the aforementioned passage orifices and a rear position where it rests on the bottom of the front housing of the piston and closes the passage through the tube running inside the tubular shank. The bushing can be moved towards the anterior position by the action of the following section of the telescopic jib through the mentioned tube, and towards the rear position by means of a recovery spring.
The external head of the shank has a pilot piston or an axially displaceable piston which may be activated by the following section of the telescopic jib in its retraction position. This pilot or displaceable piston causes a partial displacement of the tube running inside the tubular shank, provoking the displacement of the bushing fitted over the cover conduit towards its anterior position.